Cemented carbides, which are often used in cutting tools according to their super hard nature, contain expensive rare elements such as tungsten, cobalt, tantalum, and niobium. These elements are desired to be recovered as much as possible from scrap.
The methods for recycling scrap of carbide tools are broadly classified into two groups: direct methods in which solid scrap is recycled into powder with the composition maintained; and indirect methods in which scrap is chemically dissolved and then the constituents are each separately recovered.
A typical example of the direct methods is zinc treatment (see, for example, Japanese Examined Patent Application Publication No. 03-020445). This method allows zinc, as well as rare elements, to be recovered and recycled without using chemicals or aqueous solutions and is superior in low energy consumption. Also, this method does not require large investment for treating equipment and is thus advantageous in small-scale industrial production. The direct method, however, requires strict pre-screening because the composition of scrap is kept even after recovery. This increases screening cost and cancels the benefit in terms of process cost. In addition, in recovery of cutting tools that account for a major part of production, the constituents of the coatings of the cutting tools are also collected at the same time. This is a cause of degradation in quality of recycled materials. Accordingly, such a recycled material is limited in use and, for example, must be mixed in a virgin powder before use. This is a large problem. It is an issue to develop applications in which the recycled materials can be used in a wide range.
In wet chemical treatment that is an indirect method, the step of recycling scrap is generally performed in an ore refining process (see, for example, Japanese Unexamined Patent Application Publication No. 2004-002927). For cemented carbide scrap, which mainly contain WC and Co, an aqueous solution of Na2WO4 is prepared by alkaline extraction or alkaline dissolution, and impurities other than tungsten such as Co, Ti, Ta, Nb, Cr, and V, which have been added to the cemented carbide as additive elements, are removed by being precipitated as insoluble compounds by pH adjustment or sulfuration, and filtering out the insoluble compounds, as shown in FIG. 3. Tungsten is separated and extracted from the filtered Na2WO4 aqueous solution. To the tungsten, ammonia is added, and ammonium paratungstate (APT) is crystallized by heating and concentration. The APT is purified into tungsten oxide by pyrolysis.
Alkaline extraction is a method for extraction from previously oxidized and roasted scrap using a NaOH solution, or an alkaline solution, and is suitable for treating powdery soft scrap such as grinding sludge. On the other hand, alkaline dissolution is a method performed by simultaneously oxidizing and dissolving scrap using a molten sodium salt such as NaNO3, Na2SO4, or Na2CO3, and is suitable for treating solid hard scrap.
A known method for obtaining, for example, tungsten oxide using alkaline dissolution is performed in the following process:
(1) Na2WO4 prepared by dissolution in a molten salt is dissolved in water to yield a Na2WO4 aqueous solution. In an ore refining process, SiO2, As, P, Mo, and other impurities contained in ore are dissolved in this step. These are removed by being precipitated by pH adjustment or sulfuration and filtered out. For recycling waste carbide tools, impurities other than tungsten such as Co, Ti, Ta, Nb, Cr, and V which have been added to cemented carbide as additive elements are removed by being precipitated as insoluble compounds by pH adjustment or sulfuration, and filtering out the insoluble compounds.
(2) To the Na2WO4 solution, CaCl2 is added to precipitate CaWO4, and the CaWO4 slurry is rinsed with water to remove the Na component.
(3) To the CaWO4 slurry, HCl is added to precipitate tungstic acid (H2WO4), and the H2WO4 slurry is rinsed with water to remove Ca ion.
(4) To the H2WO4, NH4OH is added to prepare a (NH4)2WO4 aqueous solution.
(5) The resulting solution is heated and concentrated to crystallize ammonium paratungstate (APT).
(6) The APT is pyrolyzed into tungsten oxide (WO3).
For converting the Na2WO4 aqueous solution into a (NH4)2WO4 aqueous solution, alternatively, solvent extraction or ion exchange may be performed.
Unlike zinc treatment, wet chemical treatment can achieve high-quality recovery comparable to ore refining and is therefore advantageous in allowing recycled tungsten to be reused without limitation.